By default the pixels are the square cells at the highest level of refinement. The level of the cells displayed can be controlled using the maxlevel option of GfsOutputScalar. Setting maxlevel can also be useful to control the size (in pixels) of the image produced: when maxlevel is set the pixel size of the image will always be a multiple (depending on the number of boxes in the computation domain) of 2^maxlevel. This holds even when the number of levels in the simulation changes due to adaptive refinement (see also this question in the FAQ).

The NODATA keyword of GfsFunction can be used to mask out parts of the domain. For example drawing only the positive values of the pressure could be written: